FIG. 1 illustrates a block diagram of a prior art motor control circuit which utilizes pulse width modulation of the "on" cycle of an inverter to control the rotational velocity of a motor. A motor 10 is driven at a synchronous speed by pulses produced by an inverter 12 which have the width of the individual pulses controlled by a pulse width modulation duty cycle controller 14. The pulse width modulation duty cycle controller 14 controls the variable fundamental frequency which is produced by the inverter 12. The inverter 12 outputs a fundamental frequency which is defined by an envelope comprised of a pulse width modulated carrier signal having a frequency between 6 and 12 KHz. The pulse width modulation of the "on" interval of the individual pulses outputted by the inverter 12 results in a fundamental output signal having a desired frequency range which may be used to accelerate the motor from stop up to a predetermined velocity. The motor 10, inverter 12 and pulse width modulation duty cycle controller 14 are conventional. A resolver 16 produces an output signal which indicates the position of the rotor of the motor with respect to a reference position. The point at which the switches within the inverter are turned on with respect to a time reference to produce pulse width modulation, controls advancing or retarding of the turning on point of the inverter switches to provide control of the rotation of the rotor in accordance with a predetermined control sequence. A current feedback circuit 18 produces a signal proportional to the current flow from the switches in the inverter 12 to the motor 10. The current feedback circuit 18 was implemented by Hall sensors. The disadvantage of using Hall sensors is that inexpensive Hall sensors are prone to drift which produces a variable gain and more complex expensive Hall sensors have reliability problems. The output of the current feedback circuit 18 which is proportional to the magnitude of current flow to the motor 10 from the inverter 12 is provided as an input to a difference circuit which outputs an error signal proportional to the difference between the output signal from the current feedback circuit and a current reference produced by current reference 22. The error signal outputted by the difference circuit 20 is integrated by integrator 24 which permits the error signal to be driven to zero. A summing circuit 26 sums the output signal produced by resolver 16 and the integrator 24 to produce a resultant signal which controls the advancing or retarding of the turning on point of the switches within the inverter in a manner inversely proportional to the magnitude of the output signal produced by the summing circuit. In other words, if the integrated error signal outputted by the integrator 24 is increasing, the turning "on" points of the switches within the inverter 12 are advanced and if the integrated output error signal is decreasing, the turning "on" points of the switches within the inverter are retarded.
Additionally, current transformers have been used to sense the fundamental frequency of current driving a motor. However, the usage of current transformers to sense the frequency of current in a motor which is being started from a stop condition is unreliable for the reason that when a synchronous acceleration is used, the frequency of the drive current at the time of starting the motor from a stop condition is so low that the operation of the current transformers does not produce a signal proportional to the magnitude of the current flow. Furthermore, current transformers have been used to sense the magnitude of current flowing in an inverter which is being pulse width modulated for the purpose of shutting down the inverter when the current flow exceeds a maximum permissible current.